gacha

It's a mythical reenactment of the Chaoskampf/Drakenkampf theme universal for many mythos, depicting a culture hero/deity defeating a chaos monster, often related to the sea and in a form of a serpent or a dragon
Marduk defeating Tiamat, Hadad defeating Lotan, Yahweh defeating Leviathan, Zeus defeating Typhon, Thor's struggle against Jormungandr, Horus fighting Sep, Ra fighting Apophis, Sigurd/Siegfried slaying Fafnir, Saint George slaying dragon etc.
The Watcher is that chaos monster and all but outright confirmed to be Lotan/Leviathan, derived from Tiamat (one of the Watcher's skills is Tiamat's Authority)
And because Watcher is derived from Tiamat and Tiamat is the Goddess of Creation, hence the "pierce all of creation" line, because Sigma must ultimately defeat Watcher